Various attempts have been made to provide sensing capabilities in the context of petroleum exploration, production, and monitoring, with varying degrees of success. Recently, these attempts have included the use of fiber optic cables to detect acoustic energy. Because the cables typically comprise optically conducting fiber containing a plurality of backscattering inhomogeneities along the length of the fiber, such systems allow the distributed measurement of optical path length changes along an optical fiber by measuring backscattered light from a laser pulse input into the fiber. Because they allow distributed sensing, such systems may be referred to as “distributed acoustic sensing” or “DAS” systems. One use of DAS systems is in seismic applications, in which seismic sources at known locations transmit acoustic signals into the formation, and/or passive seismic sources emit acoustic energy. The signals are received at seismic sensors after passing through and/or reflecting through the formation. The received signals can be processed to give information about the formation through which they passed. This technology can be used to record a variety of seismic information. Another application range is concerning in-well applications.
While there exists a variety of commercially available DAS systems that have varying sensitivity, dynamic range, spatial resolution, linearity, etc., all of these systems are primarily sensitive to axial strain. As the angle between direction of travel of the acoustic signal and the fiber axis approaches 90° , DAS cables become much less sensitive to the signal and may even fail to detect it.
Thus, it is desirable to provide an improved cable that is more sensitive to signals travelling normal to its axis and enables distinguishing this radial strain from the axial strain. Such signals may sometimes be referred to as “broadside” signals and result in radial strain on the fiber. Sensitivity to broadside waves is particularly important for seismic or microseismic applications, with cables on the surface or downhole.